Highly efficient and chemoselective ruthenium-catalyzed hydrosilylation of aldehydes

Article abstract of DOI:10.1002/adsc.201100426

The highly chemoselective hydrosilylation of aldehydes was achieved using a ruthenium catalyst activated by a household fluorescent light (30 W) at or below room temperature. The hydrosilylation was almost exclusive to aldehydes over ketones and olefins.

Full text of DOI:10.1002/adsc.201100426

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DOI: 10.1002/adsc.201100426
Highly Efficient and Chemoselective Ruthenium-Catalyzed
Hydrosilylation of Aldehydes
Youngshil Do,a Junghoon Han,a Young Ho Rhee,a,* and Jaiwook Parka,*
a
Department of Chemistry, Pohang University of Science and Technology (POSTECH), San 31 Hyojadong, Pohang,
Kyeongbuk 790-784, Republic of Korea
Fax : (+82)-54-279-3399; e-mail: yhrhee@postech.ac.kr or pjw@postech.ac.kr
Received: May 25, 2011; Revised: July 31, 2011; Published online: December 6, 2011
This paper is dedicated to Professor Eun Lee on the occasion of his honorable retirement and 65th birthday.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100426.
Abstract: The highly chemoselective hydrosilylation
of aldehydes was achieved using a ruthenium cata-
lyst activated by a household fluorescent light
(30 W) at or below room temperature. The hydrosi-
lylation was almost exclusive to aldehydes over ke-
tones and olefins.
species, which has been proposed as a key catalytic
species for the racemization, might also be active for
the reduction of other functional groups. In this com-
munication, we wish to report the utility of 1 for the
chemoselective hydrosilylation of a wide range of al-
dehydes, which allows the use of various silanes under
mild conditions and does not require additives.
Keywords: aldehydes; chemoselectivity; hydrosily-
lation; photochemistry; ruthenium complexes
The hydrosilylation of carbonyl compounds is a fun-
damental transformation which has broad applications
in organic synthesis and materials science.[1] This reac-
tion is particularly attractive because it provides silyl-
protected alcohols in a single step. A number of
Initially, the reaction conditions were optimized in
metal-catalyzed protocols have been developed,[2] all the hydrosilylation of benzaldehyde (Table 1). A fluo-
aiming at the hydrosilylation of ketones.[3] On the rescent light (30 W) was needed to activate the ruthe-
other hand, protocols for the selective hydrosilylation nium complex 1 (entry 1), because the reaction car-
of aldehydes are limited. For example, the well- ried out without the fluorescent light showed no con-
known Strykerꢀs reagent [Ph3PACTHUNGRTNE(NUG CuH)]6 shows a good version (entry 2). Interestingly, however, illumination
selectivity for the hydrosilylation of aldehydes in the just for 30 min at the beginning of the reaction was
presence of ketones.[4] However, it has a strong prefer- enough to generate the catalytic species (entry 3).
ence for the competing 1,4-reductions in the hydrosi- Lowering the temperature to 08C still maintained the
lylation of a,b-unsaturated aldehydes. Recent studies catalytic activity (entry 4). Notably, the hydrosilyla-
on the chemoselective hydrosilylation of aldehydes tion showed complete conversion even at À788C with
using other metal catalysts also suffer from the low an extended reaction time (entry 5). Meanwhile, anal-
chemoselectivity, high catalyst loading, limited scope ogous diruthenium complexes such as [Cp*Ru(CO)2]2
of the silanes, requirement of additives, and harsh and [CpRu(CO)2]2 were practically ineffective for the
conditions.[5]
hydrosilylation (entries 6 and 7).
Recently, we have reported the simple synthesis of
Next, the scope of silanes was investigated in the
an isolable diruthenium complex (1) and its conver- hydrosilylation of benzaldehyde (Table 2). The reac-
sion to a catalytic species that is active for the racemi- tion with tert-butyldimethylsilane was also successful
zation of secondary alcohols simply by illuminating its to give the corresponding silyl ether in high yield, al-
solution.[6] We envisioned that the ruthenium hydride though it was slower than that with triethylsilane
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Youngshil Do et al.
Table 3. Hydrosilylation of various aldehydes.[a]
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Table 1. Catalytic hydrosilylation of benzaldehyde using 1.[a]
Entry Aldehyde
Time Product
Yield[b,c]
[h]
[%]
Entry Ru
Temp.
Time [h] Yield[b] [%]
>99
(97)
1
2
10
1
2
3
4
5
6
7
1
1
1
1
1
r.t.
r.t.
r.t.
08C
12[c]
12[d]
12[e]
12
>99
0
98
94
90
3
10
96 (94)
À788C 48
3
4
5
12
17
10
98 (96)
98 (96)
95 (95)
A
24
A
r.t.
24
0
[a]
A
solution of benzaldehyde (0.50 mmol), Et3SiH
(0.50 mmol), and 1 (0.50 mol%) in tetrahydrofuran
(THF) (0.5 mL) was illuminated with a household fluo-
rescent light (30 W) under an argon atmosphere.
Measured by GC equipped with a TR-5 column using
trans-stilbene as an internal standard.
The reaction mixture was illuminated throughout the re-
action (12 h).
The reaction was carried out in the dark.
[b]
[c]
6
7
24
24
90 (88)
96[d] (94)
[d]
[e]
8
24
96 (92)
The reaction mixture was illuminated for 30 min at the
beginning of the reaction.
9
24
24
94[e] (91)
96[d] (83)
10
Table 2. Hydrosilylation of benzaldehyde with various si-
ACHTUNGTRENNUNG
lanes.[a]
11
12
24
12
85 (75)
Yield[b] [%]
98 (93)
Entry
Silane
Time [h]
81[d,f]
(76)
85[d,f]
(83)
1
2
3
4
t-BuMe2SiH
Ph3SiH
Ph2MeSiH
24
24
24
24
93
90
13
24
36
90
13[c]
14
A
[a]
A
solution of benzaldehyde (0.50 mmol), a silane
[a]
Typical procedure: A solution of aldehyde (0.50 mmol),
Et3SiH (0.50 mmol), and 1 (0.50 mol%) in THF (0.5 mL)
was illuminated with a fluorescent light (30 W) at room tem-
perature.
Measured by GC equipped with a TR-5 column using trans-
stilbene as an internal standard.
(0.50 mmol), and 1 (0.50 mol%) in THF (0.5 mL) was il-
luminated with a household fluorescent light (30 W) for
1 h at room temperature under an argon atmosphere.
Isolated yield of silyl ether.
[b]
[c]
[b]
1
Measured by H NMR using 1,2-dichloroethane as an in-
ternal standard.
[c]
[d]
Numbers in the parenthesis are isolated yields.
1
Measured by H NMR using 1,2-dichloroethane as the inter-
nal standard.
1.0 mol% of 1 was used.
Two equivalents of Et3SiH and 2 mol% of 1 were used at
808C.
(entry 1). The reactions with triphenylsilane and
methyldiphenylsilane were comparable to that with
tert-butyldimethylsilane (entries 2 and 3), while the
reaction with triisopropylsilane was much less effi-
cient (entry 4).[2b]
[e]
[f]
Under the optimized conditions, the hydrosilylation
using the ruthenium complex 1 and triethylsilane was
tested for various aldehydes (Table 3). The hydrosily- a,b-unsaturated aldehydes (entries 6 and 7).[7] Ali-
lation of aromatic aldehydes was not affected signifi- phatic aldehydes were also good substrates for the hy-
cantly by the ring substituents except for a nitro drosilylation, although the reactions were slower than
group (entries 1–5).[7] Chloro and ester groups re- those of aromatic ones (entries 8–11). Interestingly,
mained intact during the hydrosilylation (entries 2 the steric effect of the alkyl groups was not signifi-
and 3). ortho-Methyl groups appeared to slow the hy- cant, as shown by the effective reaction of isobutyral-
drosilylation, while ortho-methoxy groups did not dehyde (entry 10) and pivalaldehyde (entry 11). The
show retarding effect (entries 4 and 5). Notably, only furan ring did not interfere with the catalytic hydrosi-
1,2-addition was observed in the hydrosilylation of lylation (entry 12). Also, the hydrosilylation of thio-
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Highly Efficient and Chemoselective Ruthenium-Catalyzed Hydrosilylation of Aldehydes
phenyl- and pyridylaldehydes was successful with a ruthenium hydride (D). The reaction of a trialkylsi-
higher catalyst loading under elevated temperature lane with E produces the silyl ether product with re-
(entries 13 and 14).
generation of D. In fact, the ruthenium hydride B was
Most notably, this catalytic system showed remark- observed in the reaction mixture during the hydrosily-
able chemoselectivity towards aldehydes over ke- lation.[12,13] In a separate experiment, we confirmed
tones. For example, 3-acetylbenzaldehyde was almost that the catalytic hydrosilylation can be effected using
exclusively transformed into the corresponding pri- the ruthenium hydride B with comparable efficiency
mary silyl ether in the reaction with an equimolar to that under the optimized conditions.[14] The inter-
amount of triethylsilane under the optimized condi- mediacy of E is also supported by the recent reports
tions [Eq. (1)].[8] Moreover, only the formyl group that suggest an analogous ruthenium alkoxide species
was reacted in the hydrosilylation of 6-oxoheptanal as the key intermediate for the catalytic racemization
[Eq (2).][9]
of alcohols.[15] Meanwhile, the direct reaction of C
with an aldehyde to give the intermediate G cannot
be ruled out, which can react with a silane to give the
silyl ether product with regeneration of C.
In summary, we have demonstrated a highly effi-
cient and practical hydrosilylation of aldehydes using
a readily available and isolable diruthenium complex
(1) under mild conditions without any additives. The
reaction is applicable to a wide range of aldehydes,
and is exclusively selective for the aldehydes over ke-
tones and olefins.
Although further studies are required to elucidate
the mechanism of the catalytic hydrosilyliation, prob-
able pathways are proposed in Scheme 1 on the basis
of the chemistry known for ruthenium complexes re-
lated to 1 and the results described above.[10] The low
energy light (l>400 nm) transforms the bridged form
into a terminal form (A).[10a,b] This initial step would
Experimental Section
General Procedure of the Hydrosilyation of
Benzaldehyde
In a J-Young flask a mixture of benzaldehyde (50 mL,
0.50 mmol), SiEt3H (80 mL, 0.50 mmol), and Ru complex 1
(2.3 mg, 0.0025 mmol) in THF (0.5 mL) was illuminated
with fluorescent light (30 W) at room temperature under
argon atmosphere. After the reaction mixture had been con-
centrated, the crude product was purified by column chro-
matography (silica gel; hexane: ethyl acetate=20:1) to give
À
be followed by the cleavage of the Ru Ru bond and
the subsequent reaction with a trialkylsilane to form a
ruthenium hydride (B) and a silyl ruthenium species
(C).[10c,11] The reaction of B with an aldehyde to form
a ruthenium alkoxide E would occur after the libera-
tion of CO from B to generate a low-coordinate PhCH2OSiEt3; yield: 110 mg (96%).
Scheme 1. Proposed pathway for the photoinduced hydrosilylation of aldehydes.
Adv. Synth. Catal. 2011, 353, 3363 – 3366
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Youngshil Do et al.
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Acknowledgements
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We are grateful for the financial support from the Korea Re-
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[7] p-Nitrobenzaldehyde, styrene, and phenylacetylene
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than 96% of the starting aldehydes remained in reac-
tion mixture after 12 h.
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